Photoluminescence Excitation Spectroscopy of Carbon-Doped Gallium Nitride
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"**Department
of Chemical Engineering, University of Wisconsin, Madison, Wisconsin 53706
Cite this article as: MRS Internet J. Nitride Semicond. Res. 4S1, G3.67 (1999) ABSTRACT We have done a comparative study of carbon-doped GaN and undoped GaN utilizing photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies in order to investigate deep levels involved in yellow luminescence (YL) and red luminescence (RL). When the GaN was excited by above-bandgap light, red luminescence (RL) centered at 1.82 eV was the dominant below-gap PL from undoped GaN, but carbon-doped GaN below-gap PL was dominated by yellow luminescence (YL) centered at 2.2 eV. When exciting PL below the bandgap with 2.4 eV light, undoped GaN had a RL peak centered at 1.5 eV and carbon-doped GaN had a RL peak centered at 1.65 eV. PLE spectra of carbon-doped GaN, detecting at 1.56 eV, exhibited a strong, broad excitation band extending from about 2.1 to 2.8 eV with an unusual shape that may be due to two or more overlapping excitation bands. This RL PLE band was not observed in undoped GaN. We also demonstrate that PL spectra excited by below gap light in GaN films on sapphire substrates are readily contaminated by 1.6-1.8 eV and 2.1-2.5 eV chromium-related emission from the substrate. A complete characterization of the Cr emission and excitation bands for sapphire substrates enables the determination of the excitation and detection wavelengths required to obtain GaN PL and PLE spectra that are free of contributions from substrate emission. INTRODUCTION The most common deep, below-gap luminescence that has been observed in GaN is the yellow luminescence (YL), a broad band centered at about 2.2 eV which appears in most published photoluminescence (PL) spectra of GaN [1-5]. The cause of the YL has been attributed to intrinsic crystal defects [3] and to impurities such as carbon [1,2], to a Ga-site vacancy and related complexes [6,7], or to a combination of several mechanisms [5]. It has also been found that the YL is far less intense in GaN grown by hydride vapor phase epitaxy (HVPE) than in GaN grown by metal organic chemical vapor deposition (MOCVD), and it should be noted that HVPE growth of GaN does not involve carbon containing source materials [8]. Another deep, below-gap PL band has been observed in GaN grown by HVPE and is centered at about 1.8 eV [9,10]. This weak red luminescence (RL) band is difficult to observe in GaN that has strong YL which may explain why RL is usually reported only from GaN grown by HVPE. Two mechanisms have commonly been suggested to explain the YL in GaN. The mechanism with the widest acceptance involves a radiative transition from a shallow donor level to a deep acceptor level with a depth of 860 meV or to a deep double donor or V a and related complexes. [2,3,6,71. A second proposed mechanism is a transition from a deep double donor to a shallow acceptor [4]. Recent experiments have suggested that there may be more than one recombination channel responsible for the YL [5], with the specific channel
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